The use of different-sized sprockets between the pedals and the drive wheel of a bicycle in order to increase the speed of bicycle has been well-known for many years in the prior art. The standard multi-speed road bicycle employs one or more main or front sprockets coupled to the pedal crank arms, and one or more smaller rear or drive sprockets coupled to the axle of rear drive wheel. A single drive chain normally links the front sprocket to the rear sprocket, allowing the rider to propel the bicycle by operating the pedals. Typically, a gear-shifting device or derailleur is associated with the front sprocket and/or the rear sprocket, to allow the rider to manually shift the chain to a differently-sized sprocket in order to change the gear ratio of the drive system.
By providing a main sprocket of substantially larger circumference than the drive sprocket, a beneficial speed increase is achieved. As the circumference of the main sprocket is increased, the speeds attainable by the bicycle rider also increase. Thus, where a high-speed bicycle is desired, it is desirable to make the main sprocket as large as possible. There is a limit, however, to the practical size of the main sprocket. The main sprocket must not be so large that it contacts the ground or the other parts of the bicycle, as this would adversely affect the overall function of the bicycle. This limit on the size of the main sprocket is especially acute in the field of bicycles known as full-suspension bicycles. Full-suspension bicycles are designed for riding on rough terrain, often at high speeds in racing conditions. These bicycles are typically fitted with shock absorbers and other devices which deflect when the bicycle encounters shocks. Thus, full-suspension bicycles require increased ground clearance because of rough terrain, and because deflection of the shock-absorbing members may bring parts of the bicycle temporarily in closer proximity to the ground than the parts of a normal"road" bicycle.
In order to achieve higher speeds despite this limit to the size of the main sprocket, some prior art bicycles have employed a pair of intermediate sprockets between the pedal sprocket and the wheel sprocket. See, for example, U.S. Pat. No. 472,796 to Moskowitz, U.S. Pat. No. 5,577,749 to Ross, and U.S. Pat. No. 5,102,155 to Chou. These bicycles employ two drive chains--one chain linking the pedal sprocket to the first intermediate sprocket, achieving a first speed increase, and a second chain linking the second intermediate sprocket to the drive sprocket, achieving a second speed increase. In this way, these bicycles are able to achieve higher gear ratios than two-sprocket, single-chain bicycles.
The four-sprocket, two-chain drive system shown in the prior art, however, is not suitable for use on certain types of bicycles. For example, full-suspension bicycles, which are designed for the heavy shocks and loads that are experienced when riding on rough terrain, cannot use the prior art four-sprocket, two-chain drive systems. This is because the full-suspension bicycle is very carefully designed around the size and position of the main sprocket and the drive chain of the drive system. As explained above, the bicycle has shock absorbers and other elements which are designed to yield in order to absorb bumps and shocks, and the acceptable amount of deflection is limited by the ground clearance required for the main sprocket. Thus, it is desirable for the main sprocket of a full-suspension bicycle to be compact.
Furthermore, the design of a full-suspension bicycle, like most bicycles, depends on a substantially constant distance between the main sprocket and the drive sprocket in order to function properly. This distance must remain fairly constant in order to keep a constant tension in the drive chain. If the rear wheel and drive sprocket were to move just slightly closer to the main sprocket, the drive chain would become loose and the drive system could cease to function properly. If the drive sprocket moves away from the main sprocket, the drive chain may become prohibitively tight, interfering with the function of the drive system and possibly breaking the chain. These problems are central to the design of a full-suspension bicycle, because although it is desirable to allow controlled deflection of various parts of the bicycle in order to absorb shocks, the distance between the main sprocket and the drive sprocket must remain substantially constant.
It is desirable for a full-suspension bicycle to have a deflection point between the rear wheel and the main frame, to absorb rear-wheel shocks. As can be seen in FIGS. 2a, 2b and 8, this is accomplished without changing the distance between the main sprocket and the rear sprocket by allowing the rear wheel forks to rotate about pivot points X and Y while the shock is absorbed by shock absorber Z.
The multi-sprocket high speed bicycle drive systems shown in the prior art are not suitable for use on such a full-suspension bicycle. For instance, the one pictured in U.S. Pat. No. 472,796 to Moskowitz shows an intermediate sprocket located above the rear wheel. Clearly, any substantial deflection of the rear wheel with respect to the main frame of the bicycle would cause unacceptable slackness in the chains of this drive system. Similarly, U.S. Pat. No. 5,102,155 to Chou shows an intermediate sprocket located between the main sprocket and the rear wheel which would also be ineffective during a deflection of the rear wheel.
The solutions disclosed in the prior art are also not easily adjustable by the bicycle user. The large intermediate sprockets must be detached and replaced with different sized sprockets in order to change the gear ratio of the drive system. Then the chains must be re-fitted, as the differently-sized sprockets will require chains of different lengths to achieve the required chain tension.
Another problem with the prior art solutions to the multiple-sprocket high-speed bicycle problem is that they are aesthetically unpleasing. Bicycle consumers and enthusiasts are accustomed to the traditional bicycle drive system's appearance, which consists of a main sprocket which is connected via a chain to a relatively smaller drive sprocket. Large intermediate sprockets clearly visible elsewhere on the bicycle frame are not consistent with this compact style.
Thus, there is a need for a high-speed bicycle drive system which uses a normally-sized main sprocket, which is aesthetically pleasing, which is easily adjustable by the bicycle user to achieve a wide variety of gear rations, and which is suitable for use with a full-suspension bicycle.